1. Field of the Invention
The invention concerns cylinder heads for internal combustion engines with metallic valve seat rings, the valve seat rings comprised of an arc wire sprayed Co/Mo base alloy, as well as an arc wire spray process with one or more metallic filled wires.
Cylinder heads are stressed in local areas, in particular at their valve seats, to the limits of their thermal or mechanical capacity. This has the consequence, that a local breakdown can occur, which can be recognized for example by cracks and/or tribologic wear.
Particularly in cylinder heads in light or heavy metal engineering, valve seat rings of enduring material are form-fittingly introduced by pressing into highly stressed areas. For these valve seat rings, Co and Co/Mo base alloys are of particular interest. These alloys characterize themselves by partial exceptional high hardness and very high friction wear resistance. For this reason, one of the fields of application of these alloys is in armor plating technology. A great disadvantage in this approach is the necessity to manufacture and fit the valve seat rings individually. In particular, for the very hard Co/Mo base alloys, very extensive process steps are necessary in order to achieve the required manufacturing precision.
For reducing these process steps there is proposed for example in U.S. Pat. No. 4,661,371 a process in which a protection layer is applied onto a specially prepared contact surface between valve and valve seat. This occurs by thermal spraying of a powder mixture of ceramic and metal particles, wherein the formed layer exhibits a gradiated composition, whereby a quasi ceramic cover layer is formed. The formed layers however do not exhibit optimal material characteristic.
From DE 100 41 974 A1 a process for depositing protective layers is known, in which as the powder particles a powder mixture of at least three powder components is employed. A first powder component is comprised of a Fe, Co or Ni base alloy, a second powder component is preferably formed of Mo or refractory metal sulfide and a third powder component includes preferably Cu, Al or Ag. For the illustrated process, or, as the case may be, the therewith formed layer, it is essential that the individual powder components are not melted together with each other and no alloys of the various powder components are formed. Only thereby can the individual functional characteristics (hardness, lubricity and thermal conductivity) of the employed components be retained in the layer. The powders are deposited for example by a HVOF process or by gas compacting. In the area of the highest load or stress, the hardness of the layer is in part found to be insufficient.